1. Field of the invention
The invention consists of copolymers associated with certain small molecules which form photorefractive compounds usable in optical signal processing. More specifically, it concerns new families of amorphous polymers whose lateral chains contain chromophoric groups capable of generating non-linear optical effects and electron-donor groups which, associated with electron-accepter groups, give the material its photoconductive properties.
A material is said to be "photorefractive" when illumination generates remanent variations in the refractive index. When an electric potential is applied to these materials and they are exposed to incident distributed luminosity, variations in the refractive index occur in the dark, isolating section while this variation tends towards zero in conductive illuminated zones where charge migration cancels out the voltage. When the illumination stops, the material returns to the initial situation except close to the boundary: the dielectric on the illuminated side of the boundary is depleted in photon-carriers which have migrated a certain distance and become trapped in the dark area. At the boundary, the space charges created form a local field which results in a local variation in the refractive index. This variation remains throughout the period required for dielectric relaxation of the material (several months in the case of compounds such as lithium niobate).
2. Description of the prior art
The photorefractive effect therefore records a spatial variation and is used in applications such as:
the recording of volumetric holographic networks PA1 experiments in dynamic interferometry PA1 image processing PA1 the deflection of laser beams PA1 optical amplification PA1 auto-resonant cavities PA1 optical calculation (for association memories, elementary logic components, reconfigurable interconnections). PA1 ferroelectric crystals, for example LiNbO.sub.3, BaTiO.sub.3 PA1 sillenites such as bismuth and silicon or germanium oxide crystals (Bi.sub.12 SiO.sub.20 or B.sub.12 GeO.sub.20) PA1 III-V compounds (GaAs doped with Cr, InP doped with Fe). PA1 for non-linear optical applications. Since the beginning of the 1980s, work on amorphous polymers doped with highly hyperpolarizable molecules and then amorphous copolymers in which the chromophoric molecules are directly bonded have shown that the electrooptical properties that could be obtained were comparable with those of inorganic compounds (U.S. Pat. Nos. 4,828,865 and 4,808,332, taken out by HOECHST CELANESE, the applicant's European patents EP 88 0579 and EP 89 11327). PA1 in photoconductive applications. These polymers can possess conjugated multiple bonds as in polyacetylenes and trans polyphenylacetylenes. They can include aromatic groups consisting of several benzene cores, either in the structure or in the lateral chains, for example derivatives of anthracene, pyrene or acridine. They can also possess aromatic amine functions like, for example, polyvinylcarbazole (PVK). PVK doped with 2, 4, 7 trinitro-9-fluorenone (TNF) is commercially available and has practical applications in the reproduction of documents (Xerography). These polymer materials are also generally easy to work and can be used to produce large surfaces at far less cost than inorganic materials. PA1 a group with a high .beta. hyperpolarizability value to generate strong non-linear optical effects, molecular engineering making it possible to adjust this hyperpolarizability and the transparency of the material to suit the wavelength used PA1 an electron-donor group which, in conjunction with a small electron-accepter molecule, makes the copolymer photoconductive. PA1 Y is H, CH.sub.3, Cl or F PA1 2.ltoreq.n.ltoreq.4 PA1 2.ltoreq.m.ltoreq.4 PA1 The electron-donor group D being: --O--, --S--, ##STR5## or --COO-- The electron-accepter group A.sub.e being: --CN, --NO.sub.2 ##STR6## representing a system rich in .pi.-conjugated electrons: ##STR7##
At present, most photorefractive materials are inorganic compounds such as:
However, these materials present certain disadvantages which impede further development: the compounds which offer the best performance are not available as large monocrystals and they are still very expensive. In addition, at present, none of these materials offer the qualities required, which are very fast response, high sensitivity and large memory capability.
In parallel with the development of these inorganic photorefractive compounds, organic materials, particularly polymers, have been found to possess promising properties: